Heat-sensitive recording papers provide images by utilizing physical or chemical change of a substance caused by heat energy. A considerable number of such processes have been studied.
Recently, heat-sensitive recording papers have come into use as recording papers for recording outputs from facsimile or computers utilizing their merits or primary coloration and the lack of a need for a developing step. An example of such recording papers is referred to a dye-type, and is disclosed in Japanese Patent Publication Nos. 4160/68 and 14039/70 (U.S. Pat. Nos. 2,663,654 and 2,967,785), and Japanese Patent Application (OPI) No. 27253/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") (corresponding to U.S. Pat. No. 4,283,458).
In general, the use of heat-sensitive recording paper as a recording paper is desirable because the recording devices can be made lighter in weight and smaller in size. Accordingly, such paper has rapidly come into use. On the other hand, the heat-sensitive recording paper is not desirable when chemicals or oils are deposited thereon, images recorded thereon disappear or fogs are formed. These defects are serious from the practical point of view; thus promoting efforts to eliminate such defects.
Japanese Utility Model Laid-Open No. 125354/81 proposes a means of improving resistance against color disappearance with a plasticizer by providing a coating layer of a water-soluble high polymer on a heat-sensitive color forming layer for preventing permeation of the plasticizer. However, this process is insufficient for imparting resistance against various chemicals or oils.
Japanese Patent Application (OPI) No. 146794/81 proposes a means of imparting water resistance as well as resistance against color disappearance with a plasticizer by providing a coating layer containing a hydrophobic high polymer and/or water resistance-imparting agent in addition to the water-soluble high polymer. According to this proposal, an intermolecular cross-linking agent such as formalin, glyoxal, or melamine resin is used as a water resistance-imparting agent. When a coating layer of intermolecularly cross-linked water-soluble high polymer is provided, resistance against various chemicals and oils is improved to some extent as compared to that wherein only a coating layer of water-soluble high polymer is provided, but the resistance is still insufficient.